Image data compression or expansion method and apparatus, and image transmission system and monitoring system using the method and device

ABSTRACT

Image data compression method and apparatus which improves image quality in a scene where image greatly changes and reduces the image data, and image transmission system and monitoring system using the method and apparatus. The image data compression apparatus which compresses input image data and outputs compressed image data comprises a detection unit which detects a motion or change of each scene, a determination unit which determines the detected motion amount or change amount, a switch circuit which controls a frame rate in accordance with the results of determination, and a compression encoding unit which compression-encodes the image data. The apparatus performs control such that an image having small motion amount and small change amount with respect to a predetermined image is not compression-encoded, so that the frame rate in a scene where the motion or change is large is higher than that in a scene where the motion and change are small.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a novel image data compressionor expansion method and apparatus, and more particularly, to an imagetransmission system and a monitoring system using the method andapparatus.

[0002] To transmit image data on a communication line or record theimage data in a recording device, generally, the image data must becompressed by any means because the data amount of original image datais too large. As a representative image data compression method, theH.261 method of the ITU-T (International TelecommunicationUnion-Telecommunication Standardization Sector) standards has been knownand is put into practical use.

[0003] This method compression-encodes an image signal to data of a lowbit rate of several 10 to 100 kbps, to transmit the image data on acommunication line, to realize a so-called picturephone. The methodutilizes time sequential information compression by motion compensationand spatial information compression by DCT (Discrete CosineTransformation).

[0004] Further, image data compression methods based on the JPEG, MPEG1and the like are known. These methods perform image data compressionprocessing so as to maintain a constant frame rate (the number of frameswithin a fixed period).

[0005] Generally, upon image data compression, if the content of imageis fine (many high frequency components are included), or the motion ofthe image is fast, the image data cannot be greatly compressed.Accordingly, when image data is compression-encoded to data with several10 to 100 kbps transmission speed by the above-mentioned H.261 method,generally, in a scene where the image is not so fine and the motion isslow, the frame rate (the number of frames within a fixed period) israised (increased), on the other hand, in a scene where the image isfine and the motion is fast, the frame rate is lowered (reduced).

[0006] As the frame rate is low (the number of frames is small) in ascene where the image is fine and the motion is fast, the movementdisplayed on a display device of a transmission destination isintermittent and unnatural. Further, generally, upon control to increasethe frame rate in the fast-motion scene, the image quality of the imageitself is degraded.

[0007] In this manner, with regard to a scene with large data amounteven if compressed, where the image is fine and the motion is fast, theimage data compression processing by the above-described conventionalart merely reduces the data transfer speed while degrading the imagequality of the image itself, or merely obtains a constant frame rate.The point of improvement in image quality of such scene where the imageis fine and the motion is fast has not been taken into consideration atall. That is, in the above conventional techniques cannot obtain animage with excellent image quality regarding a scene where the image isfine and the motion is fast.

SUMMARY OF THE INVENTION

[0008] Generally, in a remote monitoring system to monitor transmissionof on-site image by using a telephone line, an image with large amountof motion or change is especially significant as an image of abnormalstatus such as somebody's intrusion or fire. Further, in such remotemonitoring system, it is desired to compress and transfer even an imagewith large amount of motion or change with sufficient image quality.That is, in a scene where the motion is not fast and the amount ofchange is small, the frame rate can be low, but in a scene where theamount of change is large and the motion is fast, corresponding to anabnormal situation, it is necessary to control the transmission suchthat the frame rate is not reduced and sufficient image quality can bemaintained.

[0009] However, as described above, in the conventional technique basedon the H.261 method or the like, the frame rate is high in a scene wherethe motion is slow, which is not so significant for the monitoringsystem, while the frame rate is lowered or the image quality is degradedin a scene with fast motion, which is significant for the monitoringsystem.

[0010] For this reason, the present invention has its object to provideimage data compression or expansion method and apparatus and imagetransmission system using the method and apparatus, which solve theproblems in the conventional techniques, improve the image quality of ascene where the image greatly changes and the motion is fast, and reducethe entire image data.

[0011] Further, another object of the present invention is to apply theabove-described present invention to a remote monitoring system, toprovide an excellent remote monitoring system, in which the imagequality is not degraded in a scene where the motion is fast, significantfor the monitoring system.

[0012] According to the present invention, to attain the aforementionedobjects, the motion and change are detected in each scene, and the framerate (the number of frames within a fixed period) is controlled to behigher in a scene with large change or fast motion, than in a scene withsmall change and motion.

[0013] More specifically, the above objects are attained by providing animage data compression apparatus which compresses input image data andoutputs compressed image data, comprising: compression processing meansfor compression-processing input image data; image change detectionmeans for detecting a change on image based on the input image data;frame rate control means for controlling a frame rate of outputtedcompressed image data obtained by compression-processing image data; andmeans for controlling the frame rate of outputted compressed image datasuch that the frame rate in a scene where the image change is small islower than the frame rate in a scene where the image change is large, inaccordance with the situation of the change on image detected by theimage change detection means.

[0014] Further, according to the present invention, in the image datacompression apparatus, the image change detection means comprises changeamount detection means for detecting a change amount between images inthe input image data.

[0015] Further, according to the present invention, in the image datacompression apparatus, the image change detection means comprises motionamount detection means for detecting a motion amount on image in theinput image data.

[0016] Further, according to the present invention, in the image datacompression apparatus, the change amount detection means comprisesreference image storage means for storing predetermined image data as areference image, and arithmetic processing means for performingpredetermined arithmetic processing between the input image data and theimage data stored in the reference image storage means.

[0017] Further, according to the present invention, in the image datacompression apparatus, the arithmetic processing means is subtractionprocessing means for performing subtraction.

[0018] Further, according to the present invention, in the image datacompression apparatus, the motion amount detection means comprisesreference image storage means for storing predetermined image data as areference image and motion vector detection means for detecting a motionvector of input image data with respect to the image data stored in thereference image storage means.

[0019] Further, according to the present invention, in the image datacompression apparatus, the frame rate control means comprises switchingcontrol means for performing or not performing compression encoding onimage data in one-frame units, and the frame rate control means does notperform compression encoding on image data with a small change amount ormotion amount, by using the switching control means, in accordance withthe result of detection by the change amount detection means or that bythe motion amount detection means.

[0020] Further, according to the present invention, the image datacompression apparatus further comprises synthesizing means forsynthesizing information on the frame rate of the compressed image data,controlled by the frame rate control means, wherein the information onthe frame rate is added to the compressed image data, and the compressedimage data is outputted.

[0021] Further, according to the present invention, in the image datacompression apparatus, the information on the frame rate is informationon the number of frames thinned out for reducing the frame rate orinformation on time interval between thinned frames.

[0022] Further, according to the present invention, the image datacompression apparatus further comprises recording means for recordinginput image at predetermined time intervals, wherein the image datacompression is performed on the image recorded in the recording means.

[0023] Further, according to the present invention, the image datacompression apparatus further comprises recording means for recordingcompression-encoded compressed image data, wherein the speed ofoutputting the compression-encoded imaged data is controlled bycontrolling reading from the recording means.

[0024] Further, according to the present invention, the above objectsare attained by providing an image data expansion apparatus, whichinputs and expands compressed image data outputted from the image datacompression apparatus, comprising: decoding means for decodingcompressed image data at a frame rate controlled to be lower in a scenewhere the change is small, than in a scene where the change is large;and expansion means for expanding the compressed image data decoded bythe decoding means, wherein the input image data is reproduced such thatthe frame rate is lower in a scene where the change between images issmall than the frame rate in a scene where the change between images islarge.

[0025] Further, according to the present invention, the above objectsare attained by providing an image data expansion apparatus, whichinputs and expands compressed image data outputted from the image datacompression apparatus, comprising: decoding means for decodingcompressed image data at a frame rate controlled to be lower in a scenewhere the change is small than in a scene where the change is large;expansion means for expanding the compressed image data decoded by thedecoding means, wherein the input image data being reproduced such thatthe frame rate is lower in a scene where the change between images issmall than the frame rate in a scene where the change between images islarge; and image interpolation means for generating an interpolationimage from the input predetermined compressed image data, in accordancewith the information on the frame rate outputted with the compressionimage data, wherein the reproduced input image data is interpolated withan interpolation image generated by interpolation processing by theimage interpolation means, and the interpolated image data is outputted.

[0026] Further, according to the present invention, the above objectsare attained by providing an image data compression method forcompressing input image data and outputting compressed image data,comprising the steps of: detecting a change on image based on the inputimage data; controlling a frame rate of outputted compressed image dataobtained by compression-processing image data such that the frame ratein a scene where the image change is small is lower than the frame ratein a scene where the image change is large, in accordance with thesituation of the detected the change on image; and outputting thecompression-processed image data.

[0027] Further, according to the present invention, in the image datacompression method, detection of the change on image is made bydetecting a change amount between images in the input image data.

[0028] Further, according to the present invention, in the image datacompression method, detection of the change on image is made bydetecting an motion change amount on image in the input image data.

[0029] Further, according to the present invention, in the image datacompression method, detection of the change is made by arithmeticprocessing between images in the input image data.

[0030] Further, according to the present invention, in the image datacompression method, detection of the change is made by detecting amotion vector of the input image data.

[0031] Further, according to the present invention, the image datacompression method further comprises the step of synthesizinginformation on the controlled frame rate to the output compressed imagedata.

[0032] Further, according to the present invention, in the image datacompression method, the information on the frame rate is information onthe number of frames thinned out to reduce the frame rate or informationon time interval between thinned frames.

[0033] Further, according to the present invention, the above objectsare attained by providing an image data expansion method for inputtingand expanding compressed image data, outputted by the image datacompression method, comprising the steps of: decoding compressed imagedata, at the frame rate controlled to be lower in the scene where theimage change is small than in the scene where the image change is large;and expanding the decoded compressed image data such that the frame ratein the scene where the change between images is small is lower than thatin the scene where the change between images is large.

[0034] Further, according to the present invention, the above objectsare attained by providing an image data expansion method for inputtingand expanding compressed image data, outputted by the image datacompression method, comprising the steps of: decoding the compressedimage data at the frame rate controlled to be lower in the scene wherethe image change is small than in the scene where the image change islarge; reproducing the input image data by expanding the decodedcompressed image data such that the frame rate in the scene where theimage change is small is lower than that in the scene where the imagechange is large; and generating an interpolation image from inputpredetermined compressed image data, in accordance with information onthe frame rate, received with the compressed image data, andinterpolating the reproduced input image with the generatedinterpolation image and outputting the interpolated image.

[0035] Further, according to the present invention, the above objectsare attained by providing an image transmission system which performsimage data communication by using an image data compression apparatusand an image data expansion apparatus, comprising: the image datacompression apparatus, which compresses input image data and outputscompressed image data; transmission means for transmitting thecompressed image data, outputted from the compression apparatus, on apredetermined line; reception means for receiving the compressed imagedata transmitted via the transmission means; and the image dataexpansion apparatus, which expands the compressed image data received bythe reception means.

[0036] Further, according to the present invention, the above objectsare attained by providing a monitoring system which performs image datacommunication by using an image data compression apparatus and an imagedata expansion apparatus, comprising: image data output means forobtaining an image of a monitoring object and outputting image data; theimage data compression apparatus, which compresses input image data,from the image data output means, and outputs compressed image data;transfer means for transferring the compressed image data outputted fromthe image data compression apparatus; the image data expansionapparatus, which expands the compressed image data transferred via thetransfer means; and a display device which displays the image dataexpanded by the image data expansion apparatus.

[0037] Further, according to the present invention, in the monitoringsystem, the transfer means comprises a communication line fortransmitting image data.

[0038] Further, according to the present invention, in the monitoringsystem, the transfer means comprises a recording medium for recordingimage data.

[0039] Other features and advantages of the present invention will beapparent from the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame name or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

[0041]FIG. 1 is a block diagram showing an example of the constructionof an image compression apparatus implementing an image compressionmethod according to an embodiment of the present invention;

[0042]FIG. 2 is an explanatory view showing the principle of the imagecompression method of the present invention.

[0043]FIG. 3 is an explanatory view showing the principle of the imagecompression method of the present invention;

[0044]FIG. 4 is an explanatory view showing the principle of the imagecompression method of the present invention;

[0045]FIG. 5 is a block diagram showing another example of theconstruction of an image encoding apparatus realizing the imagecompression method according to another embodiment of the presentinvention;

[0046]FIG. 6 is a block diagram showing the schematic construction of anapplication of the image compression apparatus according to the presentinvention;

[0047]FIG. 7 is a block diagram showing the schematic construction ofanother application of the image compression apparatus according to thepresent invention;

[0048]FIG. 8 is a block diagram showing the schematic construction ofstill another application of the image compression apparatus accordingto the present invention;

[0049]FIG. 9 is a block diagram showing an example of specific circuitconstructions of change amount detector and determination unit of theimage compression apparatus;

[0050]FIG. 10 is a block diagram showing a specific construction of animage expansion apparatus according to an embodiment in relation to theimage compression apparatus in FIG. 1;

[0051]FIG. 11 is a block diagram showing the construction of the imageexpansion apparatus according to an embodiment in relation to the imagecompression apparatus in FIG. 5;

[0052]FIG. 12 is a block diagram showing an application of the imageexpansion apparatus in FIGS. 10 and 11;

[0053]FIG. 13 is a block diagram showing another application of theimage expansion apparatus in FIGS. 10 and 11;

[0054]FIG. 14 is a block diagram showing still another application ofthe image expansion apparatus in FIGS. 10 and 11;

[0055]FIG. 15 is a block diagram showing the construction of the imagecompression apparatus according to another embodiment of the presentinvention;

[0056]FIG. 16 is a block diagram showing the construction of the imageexpansion apparatus in relation to the image compression apparatus inFIG. 15;

[0057]FIG. 17 is a block diagram showing the construction of the imagecompression apparatus according to still another embodiment of thepresent invention;

[0058]FIG. 18 is a block diagram showing the construction of the imageexpansion apparatus in relation to the image compression apparatus inFIG. 17;

[0059]FIG. 19 is a block diagram showing the construction of a remotemonitoring system using the image compression apparatus and the imageexpansion apparatus according to the present invention;

[0060]FIG. 20 is a block diagram showing another example of the remotemonitoring system using the image compression apparatus and the imageexpansion apparatus according to the present invention; and

[0061]FIG. 21 is a graph showing the relation between data amount andframe rate of image data compression-encoded by the image compressionmethod according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0062] Hereinbelow, preferred embodiments of the present invention willnow be described in detail in accordance with the accompanying drawings.

[0063]FIG. 1 is a block diagram showing an example of the constructionof an image compression apparatus implementing an image compressionmethod according to an embodiment of the present invention. In FIG. 1,reference numeral 100 denotes an image compression apparatus; 101, aninput terminal for inputting digital image data; 102, a switch circuit;103, a reference image memory for storing image data as a reference forat least one frame; 104, a switch circuit; 105, an input image memoryfor storing an input image for at least one frame; 106, a detector whichcompares an input image with a reference image and detects motion amountand change amount; 107, a determination unit which compares the motionamount and change amount detected by the detector 106 with predeterminevalues, and determines whether or not the input image is encoded orwhether or not it is communicated; 108, a switch circuit; and 109, acompression encoder which compression-encodes image data. Note that thecompression encoder 109 performs data compression by the JPEG or MPEG1method, for example. Further, numeral 110 denotes an output terminal foroutputting compressed image data. In FIG. 1, the switch circuit 102, thereference image memory 103, the switch circuit 104, the input imagememory 105 and the detector 106 construct motion-amount and changeamount detection means 111; and the determination unit 107 and theswitch circuit 108 construct frame rate control means 112.

[0064] Prior to description of the operation in FIG. 1, the principle ofimage data compression of the present invention will be described withreference to the explanatory views of FIGS. 2 to 4.

[0065] In FIG. 2, numerals 201 to 203 denote three images continuouslyobtained at predetermined time intervals (e.g., {fraction (1/30)} sec.The time interval may be 1 sec., or may not be constant interval). Inthese images 201 to 203, numeral 204 denotes an object (a vehicle inFIG. 2) in each image.

[0066] In FIG. 2, when the image changes from 201 to 202, the object 204almost does not move (it exists around a left end of the image),however, when the image changes from 202 to 203, the object 204 movesfrom the left end to the right end of the image. According to the imagecompression method of the present invention, when this image istransmitted, as the image 202 almost does not change (the change amountis small) with respect to the image 201, the image 202 is notcompression-encoded and is not transmitted.

[0067] On the other hand, in the image 203, as the change (changeamount) is larger in comparison with the image 201, the image 203 iscompression-encoded and is transmitted. When the image 203 istransmitted, the image 203 itself may be compression-encoded andtransmitted, otherwise, only a portion changed with respect to the image201 may be transmitted or only the changed portion may becompression-encoded and transmitted. Note that it is significant thatthe image 202, with small change and motion with respect to the image201, is not compression-encoded and is skipped (thinned out).

[0068] Next, FIG. 3 shows an example where an object moving in the imageis small in comparison with the example in FIG. 2. In FIG. 3, numerals301 to 303 denote continuously obtained three images similar to those inFIG. 2. Numeral 304 denotes an object (an airplane in FIG. 3) in eachimage.

[0069] In this example, similar to FIG. 2, when the image changes from301 to 302, the object 304 almost does not move, however, when the imagechanges from 302 to 303, the object 304 moves greatly from the left endto the right end of the image. However, the object 304 (airplane) in theimage in FIG. 3 is smaller than the object 204 (vehicle) in FIG. 2, andin comparison between the images 301 and 303, the change amount withrespect to the entire image is not so large. In this case, the motionamount (i.e., the speed of motion) of the object 304 within a fixedperiod (it may be simply the interval between the respective images) isdetected. By detecting the moving amount of the object on the image,although the change amount is small in comparison between the image 302and the image 303, the motion amount is large (the motion is fast) incomparison between the image 301 and the image 303. In this case, theimage compression method of the present invention does notcompression-encodes nor transmits the image 302, but compression-encodesthe images 301 and 303 and transmits these images. Note that when theimage 303 is transmitted, a so-called motion vector of the object 304may be transmitted. That is, the “change” here means the change amountbetween images and the motion amount on an image.

[0070] Further, FIG. 4 shows continuously obtained 10 images, similar tothose in FIGS. 2 and 3. As it is apparent from this figure, the changeamount is small from an image 401 to an image 405, then the changeamount is large or the motion is fast from an image 406 to an image 410,especially, the motion from the image 409 to the image 410 is fast.

[0071] In case of compression encoding this image by the conventionalH.261 method, for the purpose of comparison, in the image from 401 to405 with the small motion amount, the images 401, 403 and 405 arecompression-encoded, but in the image from 406 to 410 with the largemotion amount, only the image 408 is compression-encoded. Accordingly,the frame rate of the outputted image signal is controlled to be high inthe scene with small motion amount and lower in the scene with largemotion amount.

[0072] On the other hand, in the present invention, as shown in FIG. 4(i.e., “◯” represents compression encoding and “×” represents skipping(thinning out)), when the image 401 has been encoded, as there is almostno motion in images 402 to 404, these images are skipped (thinned out),and the image 405 is compression-encoded. Further, the images 406 and408 are skipped (thinned out), and the images 407, 409 and 410 areencoded.

[0073] In this manner, in comparison with the case where 10 images aresimply transmitted, a smaller number of images (5 in this case) aretransmitted, accordingly, the transmission data amount can be reduced.Further, as the frame rate of the image with fast motion can be set tobe high, when this image is decoding-expanded, omission of frame ofsignificant image with motion or change can be prevented, rather, thesignificant image frames are increased. That is, in the image with fastmotion, the motion or change can be treated in detail withoutdegradation of image quality.

[0074]FIG. 21 is a graph showing the relation of data amount and theframe rate of image data compression-encoded by the image compressionmethod according to the present invention. In the graph of FIG. 21, thehorizontal axis represents the data amount per frame, and the verticalaxis, the frame rate. A solid line A represents the relation between theamount and frame rate of image data compression-encoded by the imagecompression method according to the present invention. Further, for thepurpose of comparison, a dashed line B represents the relation betweenimage data and its frame rate obtained by the conventional technique(H.261 method), and an alternate long and short dashed line C representsthe relation between image data amount and frame rate obtained by theother conventional technique MPEG1.

[0075] That is, as apparent from the graph of FIG. 21, as the change inimage increases, the data amount of the compression-encoded image data,i.e., the data amount per frame increases. In the above conventionaltechnique (H.261 method), the frame rate is reduced as represented bythe dashed line B. In the relation between image data amount and framerate obtained by the MPEG1, as the image quality of thecompression-encoded image is degraded, the frame rate is controlled tobe constant regardless of data amount of the image data, as representedby the alternate long and short dashed line C.

[0076] In the relation between data amount and frame rate of image datacompression-encoded by the image compression method according to thepresent invention, as apparent from the solid line A, in an image withlarge motion amount or large change amount where the amount ofcompression-encoded image data increases, the frame rate is ratherincreased, while in an image with small motion amount and small changeamount, the frame rate is reduced.

[0077] Next, the operation of the image compression apparatus 100,having the construction described with reference to FIG. 1, will bedescribed based on the principle described with reference to FIGS. 2 to4.

[0078] In the construction of FIG. 1, initially, the reference imagememory 103 and the input image memory 105 are cleared, and the switch102 is turned on, the switch 104 is turned off, and the switch 108 isturned off. First, when a first image is inputted into the inputterminal 101, the image is stored into the reference image memory 103and the input image memory 105. At this time, as the switch 104 is off,only the first image is inputted into the detector 106, and the entireimage is regarded as a change portion. The determination unit 107determines that the change amount is large, and the switch 108 is turnedon.

[0079] As described above, when the first image has stored into thereference image memory 103 and the input image memory 105, the switch102 is turned off, and the switch 104 is turned on. Then, when thesecond image is inputted into the input terminal 101, as the switch 102is off, the image is not stored into the reference image memory 103, butstored into the input image memory 105, and inputted into the detector106. Further, the first image stored in the input image memory 105 isinputted into the compression encoder 109 via the switch 108, andoutputted to the output terminal 110. Further, the detector 106 comparesthe input second image with the first image stored in the referenceimage memory 103 via the switch 104, and detects change amount andmotion amount.

[0080] The determination unit 107 compares the detected change amountand motion amount with predetermined values, and if it determines thatthe change amount and the motion amount are small, it turns the switch108 off. In this case, the second image stored in the input image memory105 is not sent to the compression encoder 109, and as it is notcompression-encoded, it is not outputted to the output terminal 110.That is, the image frame is skipped. On the other hand, if thedetermination unit 107 determines that the change amount and motionamount are greater than the predetermined values, it turns the switch108 on. Similar to the case of the first image, the compression encoder109 compression-encodes the second image and outputs it to the outputterminal 110. In this manner, by turning the switch 108 on/off inaccordance with the change amount and/or the motion amount, the framerate is controlled. Note that the image in the reference image memory103 may be arbitrarily changed.

[0081]FIG. 9 is a block diagram showing an example of more specificcircuit constructions of the detector 106 and the determination unit 107in FIG. 1. That is, in FIG. 9, numeral 901 denotes an input imageinputted from the input terminal 101; 902, a reference image stored inthe reference image memory 103; and 903, a subtracter which obtainsdifference between the input image 901 and the reference image 902. Thesubtracter 903 calculates difference with respect to image data in pixelunits. Further, numeral 904 denotes an arithmetic unit which performspredetermined calculation. The arithmetic unit 904 may obtain an averagevalue of the entire image frame from the subtracter 903, for example.Otherwise, the arithmetic unit 904 may obtain a squared average value ofthe entire image frame from the subtracter 903, or may obtain theaverage value by any other calculation.

[0082] Further, numeral 905 denotes a comparator which determines thevalue obtained by the arithmetic unit 904; and 906, a reference valuesetting unit which sets a reference value for the comparator 905. Forexample, if the comparator 905 determines that the value from thearithmetic unit 904 is greater than the reference value set by thereference value setting unit 906, it determines that the change amountof the input image 901 is large.

[0083] Note that the embodiment as shown in FIG. 1 transmits only imagedata compression-processed from an input original image. However, thepresent invention is not limited to this arrangement. For example, asshown in FIG. 5, compressed image data, with additional information suchas information on the number of skipped images and time interval, can beoutputted.

[0084] Note that in FIG. 5, the elements corresponding to those in FIG.1 have the same reference numerals, and the explanations of theseelements will be omitted. In FIG. 5, numeral 501 denotes a synthesizingunit which synthesizes image data with additional information from thedetermination unit 107. Note that the additional information includes,e.g., information as to whether or not image data to be transmitted isreference image data, information on compression method, information onthe number of skipped images, the time interval between the image datato be transmitted and previously transmitted image data, or the like.The synthesizing unit 501 synthesizes the additional information withthe image data and outputs the data to the output terminal 110.

[0085] Next, an application of the image compression apparatus 100according to the present invention will be described with reference toFIGS. 6 to 8.

[0086]FIG. 6 is a block diagram showing an example where the imagecompression apparatus 100 of the present invention is used in an imagetransmission system, applied to a remote monitoring system. In FIG. 6,numeral 601 denotes a video camera installed in a monitoring site. Notethat the output from the video camera (original image) is inputted intoa storage device 602. Thus, the image obtained by the video camera 601is stored in one-second intervals. Note that the storage device 602 maycomprise a semiconductor memory, or a video tape recorder, disk recorderor the like. Further, in FIG. 6, numeral 603 denotes a communicationinterface (modem) for communicating image data compressed by the imagecompression apparatus 100 of the present invention on a communicationline. Numeral 604 denotes a communication line.

[0087] Note in the above construction, the communication line 604 canbe, e.g., an analog telephone line or a digital ISDN line. Especially,if the communication line 604 is an analog telephone line, thecommunication interface 603 comprises a modem or the like to performsignal modulation and demodulation.

[0088] Further, the output (original image) from the video camera 601may be stored into the storage device 602, constantly at fixed timeintervals. Otherwise, it may be arranged such that when an abnormalityin monitoring (i.e., any motion on the image as the object ofmonitoring) is detected from the image obtained by the video camera 601,the image is stored. Further, it may be arranged such that a sensor orthe like for abnormality detection (e.g., a sensor which detects smokeor fire in remote fire monitoring system) is provided in addition to thevideo camera 601, and when the sensor detects an abnormality, the imageis stored.

[0089] In the remote monitoring system having the above construction,when an image stored in the storage device 602 as above is monitored ina remote place via the communication line 604, in case of compressingand transmitting the image stored in the storage device 602 by oneframe, as in the conventional manner, if the communication line 604 isan analog telephone line, it takes much time to transmit the imagebecause the communication speed is low. Further, as the storage device602 merely holds images simply inputted at fixed time intervals, animage with almost no change, which is not necessary for monitoring, isalso sent.

[0090] Generally, an image which needs monitoring in the monitoringsystem, i.e., an image especially significant for the purpose ofmonitoring, different from a normal monitoring image, has a changeportion which may be an abnormality. According to the remote monitoringsystem having the image compression apparatus of the present invention,as shown in FIG. 6, the image compression apparatus extracts only animage (frame) with large change amount or large motion amount fromimages stored in the storage device 602, then compression-encodes onlythe extracted image (frame), and transmits it via the communication line604. In other words, the image compression apparatus 100 performscompression encoding and transmission such that in a scene where theimage changes greatly or the motion is fast, the frame rate (the numberof frames transmitted within a fixed period) is not greatly reduced,while in a scene where the change amount and the motion amount aresmall, the frame rate is reduced greatly.

[0091] By this arrangement, as only an image (frame) necessary formonitoring is extracted from images stored in the storage device 602,the image quality of a motion-fast image is not degraded, and the entireimage can be quickly transmitted via a communication line with lowcommunication speed such as an analog telephone line, by reduction ofimage data amount. Note that if the communication speed of thecommunication line is not so high, as in this example, a storage devicefor storing compression-encoded image data may be provided at a previousstage of the communication interface, and data reading may be controlledfor leveling the speed to output data onto the line.

[0092]FIG. 7 is a block diagram showing an another application where theimage compression apparatus 100 of the present invention is used in animage transmission system using a LAN of the Ethernet or the likesufficiently fast for transmitting digital image data, or a high-speedcommunication line, and is applied to a remote monitoring system. Notethat in FIG. 7, numeral 701 denotes a video camera; 702, a lineinterface; and 703, a communication line such as an Ethernet LAN orhigh-speed digital line.

[0093] In this case, the video camera 701 sends one image (one frame)in, e.g., {fraction (1/30)} sec units, to the image compressionapparatus 100. The image compression apparatus 100 performs compressionencoding by the above-described method such that the frame rate (thenumber of frames within a fixed period) of a scene where the imagegreatly changes or the motion is fast is not greatly reduced, while theframe rate of a scene where the change amount and the motion amount aresmall is greatly reduced, and transmits the images via the lineinterface 702 on the communication line 703.

[0094] By this arrangement, the image quality of a motion-fast image isnot degraded, and the entire image can be quickly transmitted, while theimage data amount is reduced, even on a communication line with a lowcommunication speed such as an analog telephone line. Further, astransmission is performed with emphasis on an image significant inmonitoring, and the traffic (data transmission amount) on thecommunication line 703 can be reduced markedly in comparison with theconventional transmission, a larger number of cameras and the like canbe connected to a communication line of the same capacity.

[0095]FIG. 8 is a block diagram showing another application where theimage compression apparatus 100 of the present invention is applied to amonitoring system in which a monitoring image is not transmitted on acommunication line as in FIGS. 6 and 7, but is stored in a video tape, amagnetic disk, an optical disk, a large capacity semiconductor memory orthe like. In FIG. 8, numeral 801 denotes a video camera; 802, a largecapacity recording device which may comprise a semiconductor memory, avideo tape recorder, a hard disk device, a disk recorder or the like.

[0096] In the monitoring system of this example, similar to the abovecase, the image compression apparatus 100 of the present inventioncontrols the frame rate to be high in a scene where the image greatlychanges or the motion is fast. In other words, as only a frame withlarge change amount and a frame with fast motion are recorded, onlysignificant frames for the purpose of monitoring can be recorded.Further, the image quality of motion-fast image is not degraded, and theentire image can be transmitted even on a communication line with a lowcommunication speed such as an analog telephone line, while the imagedata amount is reduced. Accordingly, if a storage medium of the samecapacity as that in the conventional recording is used in imagerecording, the recording can be performed for a period far longer thanthat in recording in the conventional recording.

[0097] In FIGS. 1 to 9, the apparatus realizing the image datacompression method of the present invention and image transmissionsystem and monitoring system using the apparatus have been described.Next, an embodiment of image expansion apparatus in relation to theabove-described image compression apparatus will be described withreference to FIG. 10.

[0098]FIG. 10 shows an example of the construction of an image expansionapparatus in relation to the image compression apparatus in FIG. 1. InFIG. 10, numeral 1000 denotes an image expansion apparatus according tothe present invention; 1001, an input terminal for inputting image datacompression-encoded in accordance with the present invention; 1002, adecoder which decodes coded data; 1003, an expander which expandsdecoded compressed image data; and 1004, an output terminal foroutputting expanded original image data (original image). Note that ifthe compression encoder 109 in FIG. 1 compresses image data by the JPEGmethod, the decoder 1002 decodes image data by Huffman decoding, thenthe expander 1003 performs inverse quantization or inverse DCT on theimage data, to reproduce original image data.

[0099] That is, the image expansion apparatus performs decodingexpansion on image data, transmitted such that the frame rate of animage where the motion is fast (motion amount is large) is high (thenumber of frames per a fixed period is large), while the frame rate ofan image where the motion is slow (motion amount is small) is low (thenumber of frames per a fixed period is small). Accordingly, omission ofsignificant image with motion or change can be prevented, rather, thenumber of such significant images is increased. That is, the motion orchange can be treated in detail without degrading image quality ofmotion-fast image.

[0100]FIG. 11 is a block diagram showing the construction of the imageexpansion apparatus according to an embodiment, in relation to the imagecompression apparatus in FIG. 5. In FIG. 11, numeral 1101 denotes aninput terminal; 1102, a separating unit which separates data into imagedata and additional information; 1103, decoding expander which decodesand expands compressed image data to reproduce original image data;1104, a switch; 1105 and 1106, a first reproduced image memory and asecond reproduced image memory for storing reproduced image data; 1107,an interpolation processor which performs interpolation processing basedon the image stored in the first reproduced image memory 1105 and theimage stored in the second reproduced image memory 1106 and obtains aninterpolation image; 1108, a switch; and 1109, an output terminal.

[0101] In the image expansion apparatus, when image datacompression-processed by the image compression apparatus shown in FIG. 5and additional information are synthesized and inputted into the inputterminal 1101, the separating unit 1102 separates the input data intothe image data and the additional information, and the decoding expander1103 reproduces original image data. In a case where the second image(frame) is skipped, and the first and third images arecompression-processed and inputted, the first image is stored via theswitch 1104 into the first reproduced image memory 1105, and the thirdimage is stored via the switch 1104 into the second reproduced imagememory 1106. Then, the interpolation processor 1107 performsinterpolation processing based on the first and third images, toreproduce an image corresponding to the skipped second image, andoutputs the images, via the switch 1108 switched for the respectiveimages, to the output terminal 1109.

[0102] In this manner, although the image compression apparatus (100 inFIG. 5) can reduce the amount of transmission data by skipping an image(frame), the image expansion apparatus shown in FIG. 11 can reproducethe skipped image (frame) by interpolation processing and display thereproduced image. Accordingly, omission of image frame with motion orchange can be prevented, and degradation of image quality of motion-fastimage can be prevented. Further, even a motion-slow image frame can bereproduced by interpolation, and displayed at a normal frame rate, thus,the image quality of reproduced and displayed image can be improved, inused of data of the same transmission amount.

[0103] Further, FIGS. 12 to 14 show specific applications of the imageexpansion apparatus in FIGS. 10 and 11. Note that in FIGS. 12 to 14,numeral 1000 denotes the image expansion apparatus shown in FIG. 10 orFIG. 11.

[0104] First, FIG. 12 shows the image expansion apparatus in relation tothe image compression apparatus FIG. 6. In FIG. 12, numeral 1201 denotesa telephone line; 1202, a modem; 1203, a recorder for recording animage; and 1204, a display such as a CRT, a plasma display or a liquidcrystal display, for monitoring an image. As shown in FIG. 12, in a casewhere the telephone line 1201 is used as a image data transmission path,generally, the communication speed is low. However, as described above,by the operations of the image compression apparatus and the imageexpansion apparatus, an image can be transmitted at a high speed and canbe displayed for monitoring, with image quality higher than that inconventional transmission.

[0105]FIG. 13 shows an example of the construction of the imageexpansion apparatus in relation to the image compression apparatus inFIG. 7. In FIG. 13, numeral 1301 denotes a high-speed communicationline; 1302, a communication interface; and 1303, a display.

[0106]FIG. 14 shows an example of the construction of the imageexpansion apparatus in relation to the image compression apparatus inFIG. 8. In FIG. 14, numeral 1401 denotes a reproduction device such as avideo tape recorder, a magnetic disk or an optical disk for reproducingan image recorded by the compression apparatus according to the presentinvention; and 1402, a display.

[0107] In the image expansion apparatus as shown in FIGS. 12 to 14,similar to the above example, omission of significant image frame withmotion or change can be prevented, i.e., degradation of image quality ina motion-fast image can be prevented, and the motion or change of theimage can be monitored in detail. Further, even a motion-slow imageframe can be reproduced by interpolation and displayed at a normal framerate. Accordingly, the image quality of reproduction-displayed image canbe improved, in use of image data of the same transmission amount.

[0108] Further, FIG. 15 is a block diagram showing the construction ofthe image compression apparatus according to another embodiment of thepresent invention. In the embodiment in FIG. 15, the difference betweena handled image with a reference image is obtained by subtractionprocessing to calculate a change amount, and an image with small changeamount is not compression-processed and skipped. Additional informationsuch as the number of skipped images or time is transmitted with thenext image.

[0109] In FIG. 15, numeral 1501 denotes a video camera; 1502, an A/Dconverter which converts an image signal into a digital signal; 1503, arecorder for recording a plurality of images at fixed time intervals;1504, a switch; 1505, a reference image memory for storing a referenceimage; 1506, a switch; 1507, a subtracter which performs subtractionprocessing; 1508, a difference image memory for storing a differenceimage processed by the subtracter 1507; 1509, a switch; 1510, a detectorwhich detects a change amount of an image; 1511, a determination unitwhich determines the change amount from the result of detection by thedetector 1510; and 1512, a compression encoder which performspredetermined image compression processing based on the JPEG or MPEG1method. Further, numeral 1513 denotes a synthesizing unit whichsynthesizes compressed image data with additional information from thedetermination unit 1511 (e.g., information on the number of images whichhave been skipped without compression processing because the motions ofthese images are slow, time information on the interval between skippedimages or the like); 1514, a modem which performs modulation anddemodulation for communicating the synthesized data on a telephone line;and 1515, the telephone line.

[0110] In this system of the present embodiment, images obtained by thevideo camera 1501 are stored, e.g., at 1-second intervals, in therecorder 1503, constantly for several ten frames. When an abnormality isdetected by a sensor (not shown) or by image recognition, the imagesrecorded in the recorder 1503 are transmitted via the telephone line1514. In the present embodiment, to transmit the images recorded in therecorder 1503, first, the switch 1504 is turned on, the switch 1506 isturned off, the switch 1509 is turned off, and the reference imagememory 1505 and the difference image memory 1512 are cleared.

[0111] In this status, when the first image is outputted from therecorder 1503, it is stored into the reference image memory 1505. As theswitch 1506 is off, there is no image data subtraction-processed by thesubtracter 1507. Accordingly, the first image data is also stored intothe difference image memory 1508. In this case, as the value of theentire image is treated as a difference value, the detector 1510 detectsa large change amount. The determination unit 1511 determines that thechange amount is very large, and turns the switch 1509 on.

[0112] Next, the switch 1504 is turned off while the switch 1506 isturned on, and the second image is read from the recorder 1503. At thistime, as the switch 1509 is on, the compression encoder 1512 compressesthe first image stored in the difference image memory 1508 in accordancewith the JPEG method or the like, then the synthesizing unit 1513synthesizes the compressed data with additional information, indicatingthat the compressed data is the reference image, from the determinationunit 1511, and outputs the synthesized data via the modem 1514 onto thetelephone line 1515. Regarding the second image, as the switch 1506 ison, the subtracter 1507 performs subtraction between the first imagestored in the reference image memory 1505 and the second image, andstores the obtained difference image into the difference image memory1508. If the second image does not have a large change amount incomparison with the first image, the detection value of the detector1510 is not large, then the determination unit 1511 determines thatthere is no change, and turns the switch 1509 off. That is, the secondimage is not transmitted.

[0113] Next, if the third image has a large change amount in comparisonwith the first image, a difference image as the change amount is storedinto the difference image memory 1508, then the detector 1501 anddetermination unit 1511 determine that the change amount is large, andthe switch 1509 is turned on. The compression encoder 1512 compressesthe difference images of the third and first images stored in thedifference image memory 1508 by the JPEG method or the like, and thesynthesizing unit 1513 synthesizes the images with additionalinformation indicating that the second image has been skipped. Then, thesynthesized data is transmitted via the modem 1514 onto the telephoneline 1515.

[0114]FIG. 16 is a block diagram showing the construction of the imageexpansion apparatus in relation to the image compression apparatus inFIG. 15. In FIG. 16, numeral 1601 denotes a telephone line; 1602, amodem for signal modulation/demodulation; 1603, a separating unit whichseparates data into image data and additional information; 1604, adecoding expander which decoding-expands compression-encoded image data;1605, a switch; 1606, a reference image memory; 1607, a switch; 1608, anadder; 1609, a switch; 1610 and 1611, first reproduced image memory andsecond reproduced image memory for storing reproduced images; 1612, aninterpolation processor which performs image interpolation processing;1613, a switch; 1614, a recorder for recording an image; 1615; a D/Aconverter which converts image data into an analog signal; and 1616, adisplay for monitoring an image.

[0115] In the image expansion apparatus having the above construction,the modem 1602 demodulates data sent from the telephone line 1601, thenthe separating unit 1603 separates the data into compressed image dataand additional information. The decoding expander 1604 reproducesoriginal image data from the compressed image data. If the sent imagedata is a reference image, the switch 1605 is turned on, the switch 1607is turned off, and the switches 1609 and 1613 are connected to a-side.The reproduced image data is stored into the reference image memory 1606and the first reproduced image memory 1610, then recorded in therecorder 1614, and displayed, via the D/A converter 1615, on the display1616.

[0116] Similar to the description of FIG. 15, in this image expansionapparatus, if the second image is skipped and the difference image ofthe third image is received with additional information, the separatingunit 1603 separates the data into image data and the additionalinformation. Then the decoding expander 1604 reproduces an originaldifference image from the compressed difference image data. When thedifference image data is sent, the switch 1605 is turned off, the switch1607 is turned on, the switch 1609 is connected to b-side, and theswitch 1613 is connected to b- or c-side in accordance with necessity.The adder 1608 adds the difference image, reproduced by the decodingexpander 1604, to the reference image stored in the reference imagememory 1606, so as to reproduce the third image, and stores the thirdimage into the second reproduced image memory 1611. To display only animage with large change amount, the switch 1613 is connected to thec-side. The skipped second image can be reproduced from interpolationprocessing by the interpolation processor 1612 based on the first image,reproduced and stored in the first reproduced image memory 1610, and thethird image, stored in the second reproduced image memory 1611. Whenthis second image is displayed, the switch 1613 is connected to theb-side.

[0117]FIG. 17 is a block diagram showing another example of theconstruction of the image compression apparatus, based on the MPEG 1method, according to the present invention. In FIG. 17, numeral 1701denotes an input terminal for inputting image data; 1702, a subtracter;1703, a DCT (Discrete Cosine Transformation) unit; 1704, a quantizer;1705; a variable-length encoder; 1706, a motion vector detector; 1707, alocal decoder; 1708, an inverse quantizer; 1709, an inverse DCT unit;1710, an adder; 1711, an image memory; 1712, motion compensation unit;1713, an output terminal; 1714, a switch; 1715, a motion amountdetector; 1716, a synthesizing unit; and 1717, a switch. Note that amongthese constituents, the elements denoted by references 1701 to 1713correspond to those in the image compression apparatus based on the MPEG1 method.

[0118] In this construction, initially, the image memory 1711 iscleared. When the first image data is inputted into the input terminal1701, as there is no data directed to subtraction in the subtracter1702, the DCT unit 1703 performs DCT transformation on the input data,and the quantizer 1704 quantizes the data. The quantized data isforwarded to the switch 1714 and the local decoder 1707. As the switches1714 and 1717 are initially turned on, the variable-length encoder 1705variable-length encodes the data, and outputs the encoded data, via thesynthesizing unit 1716, to the output terminal 1713. In the localdecoder 1707, first, the inverse quantizer 1708 performs inversequantization on the input data, then the inverse DCT unit 1709 performsinverse DCT transformation on the data, to restore the initial firstimage, and stores the image into the image memory 1711. When the secondimage data is inputted into the input terminal 1701, the motion vectordetector 1706 detects a motion vector with respect to the first image.

[0119] The motion compensation unit 1712 performs motion compensation onthe image in the image memory 1711 in accordance with the detectedmotion vector. Then the subtracter 1702 performs subtraction between theimage data and the second image data. That is, the difference betweenthe second image and the image obtained from motion compensation on thefirst image (schematic second image) is obtained, as a difference image.The DCT unit 1703 performs DCT transformation on the difference image,similarly to the first image, then the quantizer 1704 quantizes thedifference image, and sends the image to the switch 1714 and the localdecoder 1707. At this time, the motion amount detector 1715 compares themotion vector value (if the vector value is large, the motion is fast)detected by the motion vector detector 1706 with a predetermined value.If the vector value is less than the predetermined value, the motionamount detector 1715 determines that the motion is slow, and turns theswitches 1714 and 1717 off so as not to encode the image data and skipthe data.

[0120] As the switch 1717 is off, the image in the image memory 1711 isnot updated. If the vector value is greater than the predeterminedvalue, the motion amount detector 1715 determines that the motion isfast, and turns the switches 1714 and 1717 on. The variable-lengthencoder 1705 variable-length encodes the image data with the motionvector value. The synthesizing unit 1716 synthesizes the variable-lengthcoded data with information indicating that the image is not skipped,and outputs the synthesized data to the output terminal 1713. Note thatin the local decoder 1707, similarly to the first image, inversequantization and inverse DCT transformation are performed on thedifference image, and an image added by the adder 1710 to the imagemotion-compensated by the motion compensation unit 1712 (i.e., thesecond image) is stored in the image memory 1711.

[0121] When the third image data is inputted into the input terminal1701, the motion vector detector 1706 detects, in a case where thesecond image has been skipped, a motion vector with respect to the firstimage, and in a case where the second image has not been skipped, amotion vector with respect to the second image. The motion compensationunit 1712 performs motion compensation on the image in the image memory1711 based on the motion vector, and the subtracter 1702 performssubtraction between the motion-compensated image data and the thirdimage, to obtain a difference image. The DCT unit 1703 performs DCTtransformation on the difference image, then the quantizer 1704quantizes the image and sends the image to the switch 1714 and the localdecoder 1707. At this time, the motion amount detector 1715 compares thevector value (if the vector value is large, the motion is fast) detectedby the motion vector detector 1706 with the predetermined value. If thevector value is less than the predetermined value, the motion amountdetector 1715 determines that the motion is slow, and turns the switches1714 and 1717 off, not to perform encoding and skip the image dataagain.

[0122] Further, as the switch 1717 is off, the image in the image memory1711 is not updated. If the vector value is greater than thepredetermined value, the motion amount detector 1715 determines that themotion is fast, and turns the switches 1714 and 1717 on. Thevariable-length encoder 1705 variable-length encodes the image with themotion vector value. The synthesizing unit 1716 synthesizes thevariable-length coded data with information indicating that the imagehas not been skipped, and if the previous image has been skipped, withinformation indicating that the previous image has been skipped, andoutputs the synthesized data to the output terminal 1713. Further,information on time interval between the previously-sent image and thecurrent image (e.g., information indicative of {fraction (1/30)} sec ifthe previous image has not been skipped, while indicative of {fraction(2/30)} sec if the image has been skipped) may be sent in place of theinformation indicating that the image has been skipped/has not beenskipped. Note that in the local decoder 1707, similarly to the previousimage, inverse quantization and inverse DCT transformation are performedon the difference image, and if an image added by the adder 1710 to theimage motion-compensated by the motion compensation unit 1712 (i.e., thethird image) is not skipped, the image is stored into the image memory1711.

[0123]FIG. 18 is a block diagram showing the construction of the imageexpansion apparatus based on the MPEG 1 method, in relation to the imagecompression apparatus in FIG. 17. In FIG. 18, numeral 1801 denotes aninput terminal; 1802, a variable-length decoder; 1803, an inversequantizer; 1804, an inverse DCT unit; 1805, an adder; 1806, an imagememory; 1807, a motion compensation unit; 1808, an output terminal; anoutput terminal; 1809, a separating unit; and 1810, an imageinterpolator. Note that in FIG. 18, the elements denote by references1802 to 1807 correspond to those in the image expansion apparatus basedon the MPEG 1 method.

[0124] In the image expansion apparatus, when data compression-encodedby the image compression apparatus in FIG. 17 is inputted into the inputterminal 1801, the separating unit 1809 separates the data into imagedata and other information. The variable-length decoder 1802 decodes theseparated compressed image data, then the inverse quantizer 1803performs inverse quantization on the image data and the inverse DCT unit1804 performs inverse DCT transformation on the image data.

[0125] Note that as the image memory 1806 is initially cleared, thefirst image data passes through the image interpolator 1810 and entersthe output terminal 1808. If the second image is skipped and the thirdimage is inputted into the input terminal 1801, the separating unit 1809separates the data into image data and information indicating that oneimage has been skipped, and sends the image data to the variable-lengthdecoder 1802.

[0126] The variable-length decoder 1802 further separates the image datainto a motion vector and difference image data. The inverse quantizer1803 and the inverse DCT unit 1804 reproduce an original differenceimage from the difference image data. The motion compensation unit 1807performs motion compensation on the first image in the image memory 1806in accordance with the motion vector from the variable-length decoder1802. The adder 1805 adds the motion-compensated image to the differenceimage, to reproduce the third image. The third image is stored into theimage memory 1806, and at the same time, is sent to the imageinterpolator 1810. The image interpolator 1810 generates the secondimage by interpolation from the first and third images, if necessary,then outputs the second image to the output terminal 1808, and thenoutputs the third image to the output terminal 1808. The imageinterpolator 1810 may generate one or more images by interpolation inaccordance with information on skipped image.

[0127]FIGS. 19 and 20 are block diagrams showing a remote monitoringsystem to which the present invention is applied, for monitoring aremote place by using a telephone line. FIG. 19 shows the constructionof a terminal side which obtains an image of monitoring object andtransmits the image. FIG. 20 shows the construction of a center sidewhich displays the image, sent from the terminal side, on a monitor.

[0128] In FIG. 19, numeral 1901 denotes a video camera which performsimage pickup; 1902, a video decoder which converts an image signal fromthe video camera into digital image data; 1903, an image storage unitfor temporarily storing the image; 1904, an image compressor accordingto the present invention which compression-encodes image data. Further,in FIG. 19, numeral 1905 denotes an image memory used by the imagecompressor or the like; 1906, a telephone line for transmission; 1907, amodem for communication on the telephone line; 1908, a microphone tocollect sound; 1909, an A/D converter which converts an audio signalinto digital audio data; 1910, an audio codec which compression-encodesor decoding-expands audio data; 1911, a D/A converter which convertsaudio data into an analog audio signal; and 1912, a speaker. Further,numeral 1913 denotes a central processing unit (CPU) for overallcontrol, comprising a so-called microcomputer or the like, on theterminal side according to this example. Further, numeral 1914 denotes aROM (Read Only Memory) for storing a processing program for the CPU 1913or the like; 1915, a RAM (Random Access Memory) used as a work area orvarious buffers; and 1916, a sensor which detects an abnormality such assomebody's intrusion.

[0129] Further, in FIG. 20, numeral 2001 denotes a telephone line; 2002,a modem; and 2003, an image expander according to the present invention,which decoding-expands compressed image data. Numeral 2004 denotes animage memory used by the image expander or the like; 2005, an imagestorage unit for temporarily storing expanded image data; 2006, a videoencoder which converts image data into an analog image signal; 2007, adisplay which displays a received image; 2008, a microphone; 2009, anA/D converter which converts an audio signal into digital audio data;2010, an audio codec which compression-encodes or decoding-expands audiodata; 2011, a D/A converter which converts audio data into an analogaudio signal; 2012, a speaker; 2013, a central processing unit (CPU) foroverall control, comprising a so-called microcomputer or the like.Further, numeral 2014 denotes a ROM (Read Only Memory) for storing aprocessing program for the CPU 2013; 2015, a RAM (Random Access Memory)used as a work area or various buffers; and 2016, an operation unit foroverall operation.

[0130] In the above construction, on the transmission side in FIG. 19,when the sensor 1916 detects an abnormality, a predetermined number ofimages are obtained by the video camera 1901 at fixed time intervals(e.g., 1 sec or 0.2 sec) and stored into the image storage unit 1903.Then, the modem 1907 is used to start communication, via the telephoneline 1906, with the center machine as shown in FIG. 20.

[0131] On the other hand, the images stored in the image storage unit1903 are compression-encoded by the method as shown in FIGS. 15 and 17,and transmitted to the center machine. At this time, audio informationfrom the microphone 1908 may be compression-encoded by the audio codec1910, and transmitted with the image data in a time sequential manner.Further, it may be arranged such that the modem 1907 receives audio data(e.g., a voice threatening an intruder) sent from the center machine inFIG. 20, then the audio codec 1910 decode-expands the received data, andthe speaker 1912 outputs the data.

[0132] Then, in the center machine in FIG. 20, the modem 2002 receivesthe image data via the telephone line 2001. The image expander 2003decoding-expands the received image data by the method according to thepresent invention as described in FIGS. 11 and 18, stores the data intothe image storage unit 2005, and at the same time, displays, via thevideo encoder 2006, on the display 2007. Note that at this time,received images may be sequentially displayed, or the display screen ofthe display may be divided into N screens and images are displayed onthe N screens simultaneously. Further, similarly, the audio codec 2010decoding-expands received audio data and the speaker 2012 outputs theaudio data. Accordingly, if an operator watching the image and listeningto the sound utters words of warning toward the microphone 2008, theoperator's voice is compression-encoded by the codec 2003, and sent tothe terminal machine as shown in FIG. 19. Note that at the operationunit 2016, to instruct the way of display on the display 2007 or tomonitor the terminal machine side, start of communication can beinstructed.

[0133] Various embodiments of the present invention have been explainedas above. According to the present invention, the frame rate can becontrolled such that an image (frame) with a small change amount andmotion amount with respect to a predetermined reference image (it may bearbitrarily changed) is skipped (not compression-encoded nortransmitted), while an image with a large change amount or motion amountis compression-encoded and transmitted while avoiding skipping as muchas possible.

[0134] That is, according to the present invention, provided arereference image storage means for storing a predetermined referenceimage, change amount detection means for obtaining a difference withrespect to the reference image to detect a change amount, motion amountdetection means for detecting a motion amount with reference to thereference image, determination means for determining the change amountand the motion amount, compression encoding means forcompression-encoding image data, and switch means for on/off controllinginput to the compression encoding means in accordance with the result ofdetermination by the determination means, and control is made such thatin accordance with the change amount detection means or motion amountdetection means, an image frame with a small change amount and motionamount with respect to the reference image is not compression-encoded.

[0135] Further, recording means for recording input image data atpredetermined time intervals or recording means for recording compressedimage data compression-encoded by the above method is provided, and thespeed of outputting compressed image data encoded by the compressionencoding can be controlled by controlling reading from the recordingmeans.

[0136] As it is apparent from the detailed description as above, theimage data compression or expansion method and apparatus, and imagetransmission system and monitoring system using the method andapparatus, performs compression encoding and transmission such that theframe rate is low in a scene with small change amount and motion amount,while the frame rate is high in a scene with large change amount ormotion amount. Accordingly, especially in a monitoring system or thelike, which requires transmission of an image with a large motion amountwith high image quality, high-speed transmission with high image qualityand efficient recording, centering on such necessary image, can berealized. Further, when the transmitted and recorded image data isdecoded and monitored, the image of a significant portion with a largechange amount or motion amount can be quickly monitored with high imagequality.

[0137] As many apparently widely different embodiments of the presentinvention can be made without departing from the spirit and scopethereof, it is to be understood that the invention is not limited to thespecific embodiments thereof except as defined in the appended claims.

What is claimed is:
 1. An image data compression apparatus, comprising:compression processing means for conducting compression-processing oninput image data and outputting compressed image data; image changedetection means for detecting a change in an image of the input imagedata; frame rate control means for controlling a frame rate of thecompressed image data such that the frame rate of a scene, in which thechange in the image is small, comes to be lower than that of a scene, inwhich the change in the image is large, depending upon condition ofchanges in the image, detected by said image change detection means,wherein said frame rate control means is constructed with exchange meansfor controlling whether the input image data should be compressed andencoded by a predetermined unit, exchangeably.
 2. An image datacompression apparatus, comprising: compression processing means forconducting compression-processing on input image data and outputtingcompressed image data; image change detection means for detecting achange in an image of the input image data; and frame rate control meansfor controlling a frame rate of the compressed image data such that, ifa change in the image of input image data is smaller than apredetermined value, detected by said image change detection means, theimage inputted is skipped without compressing and encoding by means ofsaid frame rate control means.
 3. An image data compression apparatus asclaimed in claim 2, wherein said image change detection means compriseschange amount detection means for detection a change amount betweenimages of the input image data.
 4. An image data compression apparatusas claimed in claim 3, wherein said change amount detection meanscomprises reference image memory means for memorizing predeterminedimage data as a reference image; and subtraction processing means forconducting subtraction on the input image data and the image datamemorized in said reference image memory means.
 5. An image datacompression apparatus as claimed in claim 1, wherein said image changedetection means comprises change amount detection means for detection achange amount between images of the input image data.
 6. An image datacompression apparatus as claimed in claim 1, wherein said change amountdetection means comprises reference image memory means for memorizingpredetermined image data as a reference image; and subtractionprocessing means for conducting subtraction on the input image data andthe image data memorized in said reference image memory means.